An air-conditioning system for a motor vehicle typically includes a refrigerant loop having an evaporator located within a heating, ventilation, and air-conditioning (HVAC) module for supplying conditioned air to the passenger compartment, an expansion device located upstream of the evaporator, a condenser located upstream of the expansion device in front of the engine compartment, and a compressor located within the engine compartment upstream of the condenser. The above mentioned components are hydraulically connected in series within the closed refrigerant loop.
The compressor compresses and circulates a refrigerant through the closed refrigerant loop. Starting from the inlet of the evaporator, a low pressure two phase refrigerant having mixture of liquid and vapor enters the evaporator and flows through the refrigerant tubes of the evaporator where it expands into a low pressure vapor refrigerant by absorbing heat from an incoming air stream. The low pressure vapor refrigerant then exits the outlet of the evaporator and enters the compressor where it is compressed into a high pressure high temperature vapor. The high pressure vapor refrigerant then flows through the condenser where it condenses into a high pressure liquid refrigerant by releasing the heat to the ambient air outside the motor vehicle. The condensed high pressure liquid refrigerant is returned to the evaporator through the expansion device, which expands the high pressure liquid refrigerant to a low pressure mixture of liquid-vapor refrigerant to repeat the cycle.
A conventional evaporator includes an inlet manifold, an outlet manifold, and a plurality of refrigerant tubes hydraulically connecting the manifolds. Additionally, there may be one or more intermediate manifolds, such as a return manifold, between the inlet and outlet manifold. The flow rate of refrigerant through the evaporator, typically in the range of 25 to 300 kg/hr for an R-134a refrigerant, depends predominantly on the rotational speed of the engine of the motor vehicle measured in revolutions per minute (rpm). This is a result of the compressor being driven directly by the engine via an accessory belt; hence, the compressor speed changes with the engine rpm.
It is desirable to be able to aliquot, break into equal parts, the two-phase refrigerant to the refrigerant tubes of the evaporator to provide uniform cooling of the airstream. If the two-phase refrigerant enters the inlet manifold at a relatively high velocity, the liquid phase of the refrigerant is carried by momentum of the flow further away from the entrance of the inlet manifold to the distal end of the inlet manifold. Hence, the refrigerant tubes closest to the inlet manifold entrance receive predominantly the vapor phase and the refrigerant tubes near the distal end of the inlet manifold receive predominantly the liquid phase. On the other hand, if the two-phase refrigerant enters the inlet manifold at a relatively low velocity, the refrigerant tubes closest to the inlet manifold entrance receives predominantly the liquid phase and the refrigerant tubes near the distal end of the inlet manifold receives predominantly the vapor phase. This is especially true as it relates to the mass fraction of refrigerant compared to the volume fraction. In either case, this results in the misaliquoting of the refrigerant flowing through the refrigerant tube causing degradation in the heat transfer efficiency of the evaporator.
An undesirable effect of misaliquoting of the liquid refrigerant is the skewing of the temperature map of the air coming off the evaporator. At a high refrigerant flow velocity, the temperature of the air stream across the refrigerant tubes at the distal end of the inlet manifold are lower compared to that of air stream across the tubes near the inlet. At low flow velocities this is reversed. The skewing and changing pattern of temperature of outlet air is undesirable. First, it is indicative of inefficient heat transfer process. Second, it prevents appropriately locating a temperature sensor on downstream face of the evaporator. This temperature sensor is intended to measure the lowest temperature of the air and it controls the fixed displacement compressor by switching it off when a set minimum temperature is reached, thereby protecting it from being damaged. The resulting non-uniform temperature pattern, which changes subject to the refrigerant flow velocity, causes difficulty in maintaining an even balance of vent temperatures out of the HVAC module. In certain instances, this imbalance in left and right vent temperatures causes perceptible discomfort to the vehicle occupants.
There is a need for a device which regulates the aliquoting of refrigerant flow in the inlet manifold to the refrigerant tubes and maintains an even pattern of temperature of the outlet air, despite changes in refrigerant flow velocity caused by the inherently varying engine speeds.